1. Field of the Invention
The present invention relates to a semiconductor laser device having a compressive-strain active layer.
2. Description of the Related Art
M. Sagawa et al. (xe2x80x9cHigh-power highly-reliable operation of 0.98-micrometer InGaAs-InGaP strain-compensated single-quantum-well lasers with tensile-strained InGaAsP barriers,xe2x80x9d IEEE Journal of Selected Topics in Quantum Electronics, Vol. 1, No. 2, 1995, pp.189) disclose semiconductor laser device which emits laser light in the 0.98-micrometer band without employing Al in any of its constituent layers. This semiconductor laser device is formed as follows.
An n-type InGaP cladding layer, an undoped InGaAsP optical waveguide layer, an InGaAsP tensile-strain barrier layer, an InGaAs double-quantum-well active layer, an InGaAsP tensile-strain barrier layer, an undoped InGaAsP optical waveguide layer, a p-type InGaP first upper cladding layer, a p-type GaAs optical waveguide layer, a p-type InGaP second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed on an n-type GaAs substrate in this order. Next, a narrow-stripe ridge structure is formed above the p-type InGaP first upper cladding layer by conventional photolithography and selective etching, and an n-type In0.5Ga0.5P material is embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and a p-type GaAs contact layer is formed. Thus, an index-guided semiconductor laser device having a current confinement structure is realized.
The above semiconductor laser device oscillates in a fundamental transverse mode. It is reported that the reliability of the above semiconductor laser device is improved since the strain in the active layer can be compensated for. However, in the above semiconductor laser device, the characteristic temperature, which is obtained from the temperature dependence of the threshold current, is as low as 156K. Therefore, in practice, temperature control is necessary, and thus cost reduction is difficult.
In the case that a semiconductor laser device having a layering structure as described above is driven under a high output condition, dark line failure due to the optical absorption by the laser facet, which has a plurality of surface states, and a phenomenon called facet destruction, become more likely to occur. Both of these defects hinder the improvement of reliability in semiconductor laser devices, and also act as an obstacle to high output operation thereof.
A method that is commonly employed to solve the problem mentioned above and to stably operate a semiconductor laser device at high output is to reduce the optical power density at the active layer. Commonly, d/xcex93, which is the thickness of the active layer d divided by the coefficient of optical confinement xcex93, is utilized as a parameter that represents the optical power density. It can be said that the larger the value of d/xcex93, the smaller the optical power density.
If the thickness of active layer d is increased, by the increase of threshold current density, the characteristics of the semiconductor laser device drastically deteriorate. Therefore, the range within which control is possible becomes extremely narrow, thus it is not preferable as a parameter to be altered. On the other hand, it is possible to regulate the coefficient of optical confinement xcex93 by altering as parameters the thickness of the optical waveguide layer as well as the composition of the AlGaAs, which is the cladding layer, without a large deterioration in the characteristics of the semiconductor laser. Although some change occurs in the shape of a far field pattern by altering xcex93, it is known that it is possible to obtain a desired far field pattern by selecting an appropriate thickness for the optical waveguide layer and an appropriate cladding composition, at a low optical power density.
The present invention has been developed in view of the above circumstances, and an object of the present invention is to provide a semiconductor laser device in which the temperature dependence of the threshold current is improved, and which has improved reliability even at high output.
The first semiconductor laser device according to the present invention comprises: a compressive-strain active layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 (0 less than x3xe2x89xa60.4 and 0xe2x89xa6y3xe2x89xa60.1); and an optical waveguide layer made of In0.49Ga0.51P, and that lattice-matches with GaAs on both the upper and lower sides of the active layer; wherein tensile-strain barrier layers made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2P1xe2x88x92y2 (0xe2x89xa6x2 less than 0.49y2 and 0 less than y2xe2x89xa60.4) are provided between the active layer and the optical waveguide layers
In the first semiconductor laser device according to the present invention, an absolute value of a sum of a first product and a second product may be equal to or smaller than 0.25 nm, where the first product is a product of a strain and a thickness of the compressive-strain active layer, and the second product is a product of a strain of the lower and upper tensile-strain barrier layers and a total thickness of the lower and upper tensile-strain barrier layers.
The second semiconductor laser device according to the present invention comprises: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed above the GaAs substrate; a lower optical waveguide layer formed above the lower cladding layer and made of In0.49Ga0.51P which is undoped or of the first conductive type; a lower tensile-strain barrier layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2P1xe2x88x92y2 (0xe2x89xa6x2 less than 0.49y2 and 0 less than y2xe2x89xa60.4) and formed above the lower optical waveguide layer; a compressive-strain active layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 (0 less than x3xe2x89xa60.4, 0 less than y3xe2x89xa60.1) and formed on the lower tensile-strain barrier layer; an upper and formed above the compressive-strain active layer; an upper optical waveguide layer formed above the upper tensile-strain barrier layer and made of In0.49Ga0.51P which is undoped or of a second conductive type; an etching stop layer made of GaAs of the second conductive type and formed on the upper optical waveguide layer; a current confinement layer made of In0.49(Alz2Ga1xe2x88x92z2)0.51P (0.15xe2x89xa6z2xe2x89xa61) of the first conductive type and formed on the etching stop layer; a cap layer made of In0.49Ga0.51P of the first conductive type or the second conductive type and formed above the current confinement layer; an upper cladding layer of the second conductive type, formed over the cap layer; and a contact layer made of GaAs of the second conductive type and formed above the upper cladding layer. A portion of the semiconductor layer formed by the cap layer, the current confinement layer, as well as the etching stop layer are removed from one resonator facet to the other that faces it from the resonator formed by the layers described above, to a depth at which the optical waveguide layer formed of In0.49Ga0.51P which is undoped or of a second conductive type is exposed. The groove formed thereby is filled by the cladding layer of the second conductive type formed above the cap layer to form a refractive index waveguide structure. In the above semiconductor laser device, an absolute value of a sum of a first product and a second product is less than or equal to 0.25 nm, where the first product is a product of a strain and a thickness of the compressive-strain active layer, and the second product is a product of a strain of the lower and upper tensile-strain barrier layers and a total thickness of the lower and upper tensile-strain barrier layers.
With regard to the second semiconductor laser of the present invention, it is preferable that the cladding layer of the second conductive type be composed of either Alz1Ga1xe2x88x92z1As (0.6xe2x89xa6z1xe2x89xa60.8) or In0.49(Ga1xe2x88x92z3Alz3)0.51P (0.1xe2x89xa6z3 less than z2).
The third semiconductor laser device according to the present invention comprises: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed above the GaAs substrate; a lower optical waveguide layer made of In0.49Ga0.51P which is undoped or of the first conductive type formed above the lower cladding layer; a lower tensile-strain barrier layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2P1xe2x88x92y2 (0xe2x89xa6x2 less than 0.49y2 and 0 less than y2xe2x89xa60.4) formed above the lower optical waveguide layer; a compressive-strain active layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 (0 less than x3xe2x89xa60.4 and 0xe2x89xa6y3xe2x89xa60.1) formed on the lower tensile-strain barrier layer; an upper tensile-strain barrier layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2P1xe2x88x92y2 (0xe2x89xa6x2 less than 0.49y2 and 0 less than y2xe2x89xa60.4) formed above the compressive-strain active layer; an upper optical waveguide layer made of In0.49Ga0.51P which is undoped or a second conductive type formed above the upper tensile-strain barrier layer; a first upper cladding layer of the second conductive type, formed over the upper optical waveguide layer; a first etching stop layer made of InGaP of the second conductive type and formed on the first upper cladding layer; a second etching stop layer made of GaAs of either the first or second conductive type and formed on the first etching stop layer; a current confinement layer made of In0.49(Alz2Ga1xe2x88x92z2)0.51P (0.15xe2x89xa6Z2xe2x89xa61) of the first conductive type and formed on the second etching stop layer; a cap layer made of In0.49Ga0.51P of the first conductive type or the second conductive type and formed above the current confinement layer; a second upper cladding layer of the second conductive type, formed over the cap layer; and a contact layer made of GaAs of the second conductive type and formed above the second upper cladding layer. A portion of the semiconductor layer formed by the cap layer, the current confinement layer, as well as the second etching stop layer are removed from one resonator facet to the other that faces it from the resonator formed by the layers described above, to a depth at which the first etching stop layer is exposed. The groove formed thereby is filled by the second cladding layer of the second conductive type formed above the cap layer to form a refractive index waveguide structure. In the above semiconductor laser device, an absolute value of a sum of a first product and a second product is equal to or smaller than 0.25 nm, where the first product is a product of a strain and a thickness of the compressive-strain active layer, and the second product is a product of a strain of the lower and upper tensile-strain barrier layers and a total thickness of the lower and upper tensile-strain barrier layers.
With regard to the third semiconductor laser of the present invention, it is preferable that the first and second cladding layers of the second conductive type be composed of either Alz1Ga1xe2x88x92z1As (0.6xe2x89xa6z1xe2x89xa60.8) or In0.49(Ga1xe2x88x92z3Alz3)0.51P (0.1xe2x89xa6z3 less than z2).
In a semiconductor laser device of an internal confinement type with a refractive index waveguide structure that provides an internal groove as a path for a current like the second or third semiconductor laser of the present invention, in the case that the width of the groove is 1-4 xcexcm, it is preferable that the equivalent refractive index step be 1.5xc3x9710xe2x88x923-7xc3x9710xe2x88x923. In the case that the width of the groove is larger than 4 xcexcm, it is preferable that the equivalent refractive index step be greater than or equal to 1.5xc3x9710xe2x88x923. The fourth semiconductor laser device according to the present invention comprises: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed above the GaAs substrate; a lower optical waveguide layer made of In0.49Ga0.51P which is undoped or of the first conductive type formed above the lower cladding layer; a lower tensile-strain barrier layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2P1xe2x88x92y2 (0xe2x89xa6x2 less than 0.49y2 and 0 less than y2xe2x89xa60.4) formed above the lower optical waveguide layer; a compressive-strain active layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 (0 less than x3xe2x89xa60.4 and 0xe2x89xa6y3xe2x89xa60.1) formed on the lower tensile-strain barrier layer; an upper tensile-strain barrier layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2P1xe2x88x92y2 (0xe2x89xa6x2 less than 0.49y2 and 0 less than y2xe2x89xa60.4) formed above the compressive-strain active layer; an upper optical waveguide layer made of In0.49Ga0.51P which is undoped or a second conductive type formed above the upper tensile-strain barrier layer; an upper cladding layer of the second conductive type formed on the upper optical waveguide layer; and a contact layer made of GaAs of the second conductive type and formed above the upper cladding layer. Two separate grooves are formed from one resonator facet to the other that faces it from the resonator formed by the layers described above, to a depth at which the optical waveguide layer formed of In0.49Ga0.51P which is undoped or of a second conductive type is exposed. The ridge formed therebetween forms a refractive index waveguide structure. In the above semiconductor laser device, an absolute value of a sum of a first product and a second product is equal to or smaller than 0.25 nm, where the first product is a product of a strain and a thickness of the compressive-strain active layer, and the second product is a product of a strain of the lower and upper tensile-strain barrier layers and a total thickness of the lower and upper tensile-strain barrier layers.
With regard to the fourth semiconductor laser of the present invention, it is preferable that the cladding layer of the second conductive type be composed of either Alz1Ga1xe2x88x92z1As (0.6xe2x89xa6z1xe2x89xa60.8) or In0.49(Ga1xe2x88x92z3Alz3)0.51P (0.1xe2x89xa6z3 less than z2).
Note that an etching stop layer may be provided between the optical waveguide layer made of In0.49Ga0.51P which is undoped or of the second conductive type and the cladding layer of the second conductive type. In the case that such an etching stop layer is provided, it is preferable that said etching stop layer be exposed at the bottom of the groove. Further, in this case, if the cladding layer of the second conductive type is composed of Alz1Ga1xe2x88x92z1As (0.6xe2x89xa6z1xe2x89xa60.8), it is preferable that the etching stop layer be composed of In0.49Ga0.51P, and if the cladding layer of the second conductive type is composed of In0.49(Ga1xe2x88x92z3Alz3)0.51P (0.1xe2x89xa6z3 less than z2), it is preferable that the etching stop layer be composed of GaAs.
The fifth semiconductor laser device according to the present invention comprises: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed above the GaAs substrate; a lower optical waveguide layer made of In0.49Ga0.51P which is undoped or of the first conductive type formed above the lower cladding layer; a lower tensile-strain barrier layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2P1xe2x88x92y2 (0xe2x89xa6x2 less than 0.49y2 and 0 less than y2xe2x89xa60.4) formed above the lower optical waveguide layer; a compressive-strain active layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 (0 less than x3xe2x89xa60.4 and 0xe2x89xa6y3xe2x89xa60.1) formed on the lower tensile-strain barrier layer; an upper tensile-strain barrier layer made of Inx2Ga1xe2x88x92x2As1xe2x88x92y2P1xe2x88x92y2 (0xe2x89xa6x2 less than 0.49y2 and 0 less than y2xe2x89xa60.4) formed above the compressive-strain active layer; an upper optical waveguide layer made of In0.49Ga0.51P which is undoped or a second conductive type formed above the upper tensile-strain barrier layer; a first upper cladding layer of the second conductive type, formed over the upper optical waveguide layer; an etching stop layer made of InGaP of the second conductive type and formed on the first upper cladding layer; a second upper cladding layer of the second conductive type, formed on the etching stop layer; and a contact layer made of GaAs of the second conductive type and formed above the second upper cladding layer. Two separate grooves are formed from one resonator facet to the other that faces it from the resonator formed by the layers described above, to a depth at which the etching stop layer is exposed. The ridge formed therebetween forms a refractive index waveguide structure. In the above semiconductor laser device, an absolute value of a sum of a first product and a second product is equal to or smaller than 0.25 nm, where the first product is a product of a strain and a thickness of the compressive-strain active layer, and the second product is a product of a strain of the lower and upper tensile-strain barrier layers and a total thickness of the lower and upper tensile-strain barrier layers.
With regard to the fifth semiconductor laser of the present invention, it is preferable that the first and second cladding layers of the second conductive type be composed of either Alz1Ga1xe2x88x92z1As (0.6xe2x89xa6z1xe2x89xa60.8) or In0.49(Ga1xe2x88x92z3Alz3)0.51P (0.1xe2x89xa6z3 less than z2).
Further, if the first and second cladding layers of the second conductive type are composed of Alz1Ga1xe2x88x92z1As (0.6xe2x89xa6z1xe2x89xa60.8), it is preferable that the etching stop layer be composed of In0.49Ga0.51P, and if the first and second cladding layers of the second conductive type are composed of In0.49(Ga1xe2x88x92z3Alz3)0.51P (0.1xe2x89xa6z3 less than z2), it is preferable that the etching stop layer be composed of GaAs.
With regard to the fourth and fifth semiconductor lasers according to the present invention, the two grooves may be formed to the isolative positions of the device. That is, both sides of the ridge may be removed to the isolative positions of the device.
In a semiconductor laser device of provided with a ridge type refractive index waveguide structure like the fourth or fifth semiconductor laser of the present invention, in the case that the width of the ridge bottom is 1-4 xcexcm, it is preferable that the equivalent refractive index step be 1.5xc3x9710xe2x88x923-7xc3x9710xe2x88x923. In the case that the width of the ridge bottom is larger than 4 xcexcm, it is preferable that the equivalent refractive index step be greater than or equal to 1.5xc3x9710xe2x88x923.
With regard to each of the semiconductor laser devices of the present invention, it is preferable that the thickness of each of the optical waveguide layers is greater than or equal to 0.25 xcexcm.
Further, each of the semiconductor laser devices of the present invention may also be provided with a layer of Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 (x4=0.49y4xc2x10.01, 0 less than x4xe2x89xa60.3) between the compressive-strain active layer of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 (0 less than x3xe2x89xa60.4, 0xe2x89xa6y3xe2x89xa60.1) and the tensile-strain barrier layer of Inx2Ga1xe2x88x92x2As1xe2x88x92y2Py2 (0xe2x89xa6x2 less than 0.49y2, 0 less than y2xe2x89xa60.4). Said layer of Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 lattice matches with GaAs, and has a larger bandgap than the active layer.
Alternatively, a layer of Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 (x4=0.49y4xc2x10.01, 0 less than x4xe2x89xa60.3) may be provided between the tensile strain barrier layer of Inx2Ga1xe2x88x92x2As1xe2x88x92y2Py2 and the optical waveguide layer. Said layer of Inx4Ga1xe2x88x92x4As1xe2x88x92y4Py4 lattice matches with GaAs, and has a larger bandgap than the active layer.
If the lattice constant of GaAs is set to be cs, the lattice constant of the active layer is set to be ca, the film thickness is set to be da, the amount of compressive strain is set to be xcex94a, the lattice constant of the tensile-strain barrier layer is set to be cb, the total film thickness of the tensile-strain barrier layers is set to be db, and the amount of tensile strain is set to be xcex94b, then xcex94a and xcex94b are defined by xcex94a=(caxe2x88x92cs)/cs and xcex94b=(cbxe2x88x92cs)/cs, respectively. Therefore, that xe2x80x9can absolute value of a sum of a first product and a second product may be equal to or smaller than 0.25 nm, where the first product is a product of a strain and a thickness of the compressive-strain active layer, and the second product is a product of a strain of the lower and upper tensile-strain barrier layers and a total thickness of the lower and upper tensile-strain barrier layersxe2x80x9d refers to a state in which xe2x88x920.25 nmxe2x89xa6xcex94ada+xcex94bdbxe2x89xa60.25 nm
Note that xe2x80x9clattice matchxe2x80x9d refers to a state in which the absolute value of the amount of strain is less than or equal to 0.005.
The xe2x80x9cequivalent refractive indexxe2x80x9d is an equivalent refractive index in the active layer at the wavelength of oscillation in the thickness direction. In the case of the internal current confinement structure, when the equivalent refractive index of a region of the active layer located under the current confinement layer is denoted by na, and the equivalent refractive index of the other regions of the active layer located under the internal stripe is denoted by nb, the equivalent refractive index step, that is, the difference xcex94n in the equivalent refractive index is defined by nbxe2x88x92na. In the case of the ridge structure, when the equivalent refractive index of a region of the active layer which is not located under the ridge is denoted by nA, and the equivalent refractive index of the other regions of the active layer located under the ridge is denoted by nB, the equivalent refractive index step, that is, the difference xcex94N in the equivalent refractive index is nBxe2x88x92nA.
The xe2x80x9cfirst conductive typexe2x80x9d and the xe2x80x9csecond conductive typexe2x80x9d have opposite polarities from each other. For example, if the first conductive type is an n-type, then the second conductive type is a p-type.
According to the semiconductor laser devices of the present invention, since the compressive strain in the Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 active layer can be compensated for by the tensile strain barrier layers provided both above and below said active layer, defects in the active layer can be reduced. Therefore, the quality and reliability of the semiconductor laser device are improved. Further, since the bandgap between the active layer and the optical waveguide layer is great, the leakage of carriers from the active layer due to temperature variations can be suppressed, and therefore the temperature dependence of the semiconductor laser device becomes very small.
By each of the optical waveguide layers being of a thickness greater than or equal to 0.25 xcexcm, the optical power density can be reduced, and high reliability is obtained from low output to high output.
By the second conductive type cladding layers being made of Alz1Ga1xe2x88x92z1As(0.6xe2x89xa6Z1xe2x89xa60.8) or In0.49(Alz3Ga1xe2x88x92z3)0.51P(0.1xe2x89xa6Z3xe2x89xa61), the bandgap between the upper cladding layer and the active layer in each semiconductor laser device can be increased. Therefore, the characteristics of the semiconductor laser device can be improved.
Further, by the second conductive type cladding layer being made of Alz1Ga1xe2x88x92z1As and the etching stop layer being made of In0.49Ga0.51P, or the second conductive type cladding layer being made of In0.49(Alz3Ga1xe2x88x92z3)0.51P and the etching stop layer being made of GaAs, the cladding layer and the etching stop layer are respectively etched with different etchants with high selectivity. Therefore, the etching of the (second) upper cladding layer can be stopped at the upper surface of the etching stop layer with high precision, and the controllability of the stripe width is high.
By the current path in the internal or ridge stripe structure having a width of 1 to 4 xcexcm and the equivalent refractive index step, that is, the difference in the equivalent refractive index between a portion of the compressive-strain active layer located under the ridge and other portions of the compressive-strain active layer being 1.5xc3x9710xe2x88x923 to 7xc3x9710xe2x88x923, the oscillation in the fundamental transverse mode can be maintained even when output power is increased to a high level.
By the current path in the internal or ridge stripe structure having a width greater than 4 xcexcm and the equivalent refractive index step, that is, the difference in the equivalent refractive index between a portion of the compressive-strain active layer located under the ridge and other portions of the compressive-strain active layer being 1.5xc3x9710xe2x88x923 or greater, it is possible to realize a highly reliable semiconductor laser device which produces low noise even in a multimode operation.